Google updated documentation for GMC landing page requirements again. The most recent update a couple months ago specified that out-of-stock products must have a grayed-out 'add to cart' button. Now there are more options. This SEO Roundtable article has a great summary & shows exactly what was changed.

Nothing controversial; it's just detailing common sense user-friendly details: if a product is available, let people buy it. If a product is not available, or is backordered, make it obvious & include helpful info like when it's expected to ship.